Adjustable gap chemical mechanical polishing method and apparatus

ABSTRACT

A polishing apparatus for polishing a surface of wafer is provided. The apparatus includes a carrier to hold the workpiece. An array of fluid nozzles are placed across from the surface of the wafer to provide a gap between the nozzles and the surface of the workpiece. A polishing pad positioned within the gap and configured to polish the surface of the workpiece when a fluid is applied from the plurality of nozzles to push the polishing pad to the surface.

RELATED APPLICATIONS

This application claims priority from Provisional Application Ser. No. 60/484,909 filed Jul. 3, 2003 (NT-303-P), which is incorporated herein by reference.

FIELD

The present invention relates to manufacture of semiconductor integrated circuits and, more particularly, to a method and apparatus for polishing substrates.

BACKGROUND

Chemical mechanical polishing (CMP) of materials for VLSI and ULSI applications has important and broad application in the semiconductor industry. Chemical mechanical polishing is a widely used technique for planarizing metals and dielectrics as well as other types of layers on semiconductor wafers. CMP is generally used to flatten and remove material from surfaces during the wafer fabrication process, for example, during the wafer fabrication process, CMP is often used to flatten/polish the profiles that build up in multilevel metal interconnection.

In a typical CMP process, a substrate such as a semiconductor wafer is mounted on a substrate carrier, often called a head. The wafer surface is pressed against a polishing pad and moved with respect to the polishing pad. This is typically performed by rotating the wafer, moving the pad or both. The polishing pad may be a conventional polishing pad or a fixed abrasive polishing pad. Conventional or polymeric polishing pads are usually used with polishing slurries including abrasive particles and chemically reactive agents. During the CMP process, the polishing slurry is supplied onto the polishing pad as the wafer surface is pressed on the pad. The surface of a fixed abrasive polishing pad typically includes abrasive particles that are embedded in a matrix or binder material.

FIG. 1 illustrates an exemplary conventional CMP system 10 that includes a polishing pad 12 to polish a front side of the wafer 14. A wafer carrier 16 holds the wafer 14 and the polishing pad can be moved with respect to the wafer. The wafer carrier 16 may include an array of built in pressure zones 18 that are located behind the wafer. The pressure zones 18 are often formed concentrically to apply localized pressure to the backside of the wafer. By applying pressure to the selected locations of the backside, polishing rate on the corresponding locations of the front side of the wafer can be changed. During the polishing process, the wafer carrier 16 is rotated (clockwise or counter-clockwise) while the pad 12 is moved. A platen 20 with a flat surface supports the polishing pad 12. Depending on the pressure distribution profile created on the backside of the wafer, polishing rate of the corresponding regions on the wafer can be varied to achieve desired polishing on the wafer. For example, by increasing the pressure around the center of the backside, higher polishing rates are obtained at the center of the front side. However, in such systems, pressures applied by selected pressure zones onto corresponding selected locations on the wafer are not entirely independent from one another. Pressure from neighboring pressure zones may interfere with each other, which situation affects the local material removal rate and cause undesired poor or excessive material removal from the front side.

Therefore, a need exists for a chemical mechanical polishing (CMP) system that can provide accurate, stable and controllable polishing rates on a wafer.

SUMMARY

The present invention provides a polishing system using fluid from a fluid source to push a polishing pad to a workpiece surface during the polishing process. A constant gap is kept between the fluid source and the workpiece surface as the workpiece surface is polished by the polishing pad.

In one aspect of the present invention, a polishing apparatus for polishing a surface of a workpiece is provided. The apparatus includes a carrier surface configured to hold the workpiece, a plurality of fluid nozzles placed across from the surface to provide a gap between the nozzles and the surface of the workpiece, and a polishing pad positioned within the gap. The polishing pad is configured to polish the surface of the workpiece when a fluid is applied from the plurality of nozzles to push the polishing pad to the surface.

In another aspect of the present invention, a method of polishing a surface of a workpiece surface using a polishing pad is provided. The method includes the steps of placing the polishing pad within a gap defined between the surface of the workpiece and an array of nozzles, emitting a fluid from the array of nozzles to push the polishing pad onto the surface of the workpiece; and polishing the surface with the polishing pad while keeping the gap constant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a conventional chemical mechanical polishing apparatus; and

FIG. 2 is a schematic illustration of an embodiment of a chemical mechanical polishing system of the present invention.

DETAILED DESCRIPTION

The present invention provides a CMP system applying fluid flow from a fluid source to the backside of a polishing pad to cause the polishing surface of the polishing pad to be forced against a workpiece surface. During the polishing the workpiece surface with the polishing pad, the workpiece surface is kept at a predetermined distance from the fluid source, to achieve chemical mechanical polishing of the workpiece surface.

FIG. 2 depicts an exemplary CMP system 100 according to an embodiment of the invention. The system 100 comprises a fluid source assembly 102 and a carrier surface 104 to hold a workpiece 106. The workpiece 106 may be a semiconductor wafer. Carrier surface 104 may be a front surface of a wafer carrier or it may be any surface against which the backside of a wafer rests. A polishing pad 108 is positioned between a surface 110 of the wafer 106 and the fluid source assembly 102. The surface 110 of the wafer may include a conductive layer such as copper or a dielectric layer such as silicon dioxide layer to be planarized using CMP. The polishing pad 108 includes a first surface or a process surface 112 and a second surface or a back surface 114. The polishing pad 108 may preferably be tensioned by a tensioning mechanism (not shown). Process surface 112 of the polishing pad 108 polishes the surface 110 of the wafer 106 during the CMP process, typically with the help of a process solution or a polishing slurry. A variety of different polishing pads can be used with the present invention. For example, the polishing pad can be a fixed abrasive pad or a more commonly used polymeric pad. The polishing pad 108 may be used with or without a slurry. The carrier surface 104 of the system 100 may rotate or move the wafer laterally or vertically. In this embodiment, fluid source assembly 102 is placed above the wafer 106. However, other configurations which place the fluid source assembly 102 under the wafer is also possible and within the scope of this invention.

In this embodiment, the fluid may be gas such as air, or liquid such as water. During the process, a fluid flow 115 is applied to the back surface 1 14 of the polishing pad 108. The application of the fluid flow 115 to the back surface 114 of the polishing pad 108 is carried out using the fluid source assembly 102. The fluid source assembly may include a plurality of fluid nozzles 116. The fluid nozzles 116 may be arranged into any configuration or array with space 118 among them. For example, the nozzles 116 may form a nozzle array that positions nozzles a predetermined distance from one another thereby creating the space 118 among them. Alternatively, instead of leaving a space among the nozzles, holes or openings may be placed among the nozzles to remove the used fluid from the system. In this approach, the fluid flow assembly may have surface including nozzles and the openings. The nozzles 116 may form discrete zones to create a fluid flow rate distribution profile of the fluid source assembly 102. The zones may be formed concentrically and each zone may be connected to a fluid flow controller (not shown) to regulate fluid flow for each zone. In an alternative embodiment, a space or holes may exist between the zones. By varying amount of fluid flow rate from the selected fluid flow zones, a fluid flow rate distribution profile including different flow rates from different zones may be generated on the back surface 114 of the polishing pad 108. Fluid flow rate distribution profile may have high flow rate zones or low flow rate zones. Depending on the fluid flow rate distribution profile, polishing rate of the corresponding regions on the wafer 106 may be varied to achieve desired polishing on the surface 110 since more fluid flow from a given zone pushes the process surface 112 to the surface 110 with a higher force at that zone. For example, by increasing the fluid flow rate from the nozzles around the center of the fluid source assembly 102, higher polishing rates is obtained at the center of the surface 110 of the wafer.

Referring back to FIG. 2, in this embodiment, a first end 117 of the nozzles 116 are aligned with an imaginary plane P which is nearly parallel to the surface 110 of the wafer. A gap “t” is left between the first end 117 of the nozzles 116 and the surface 110 of the wafer 106. During the polishing process, the gap “t” is kept constant. The predetermined height of the gap “t” is important for the polishing process of the present invention and this height is adjustable. The gap “t” may be less than 6 millimeters. If the gap “t” is configured to be large, fluid flow rate must be high to accomplish the desired polishing rate on the surface 110 of the wafer 106. However, if the gap “t” is configured to be small, reduced flow rates can be used to accomplish desired polishing rates. The gap “t”, however, cannot be too small to allow the back surface 114 of the polishing pad 108 to touch the nozzles 116.

As shown in FIG. 2, when fluid flow 115 is applied during the process, the polishing pad 108 moves into a process position 120 within the gap “t” and is forced onto the surface of the wafer with the applied fluid flow. While the gap is kept constant or unchanged, any desired fluid flow rate distribution profile can be applied to the polishing pad 108 to obtain corresponding desired polishing rates on the surface 110. The space 118 among the nozzles 116 may be used for isolating the nozzles from the neighboring nozzles and may advantageously provide a passage or a drain for the exhausted, or used, fluid. Alternately, the used fluid may leave the system from the edges of the fluid source assembly 102. After forcing the polishing pad toward the surface of the wafer for the CMP process, the fluid flow from the nozzles 116 exits the fluid source assembly 102 through the space 118 among the nozzles 116 without interfering with the fluid flow from the neighboring nozzles. When the process is over, for example by reaching a predetermined endpoint, the fluid flow is stopped, which causes the preferably tensioned polishing pad to return to its original position within the gap “t”.

Accordingly, the present invention provides a substantially independent fluid flow for each nozzle, and if the nozzles are arranged into zones, the present invention provides distinct fluid flow rate distribution profiles. Such well-defined and independent fluid flow rate distribution profiles, in turn, establish well-defined polishing rates on the substrate as the polishing pad polishes the workpiece surface. In one embodiment, the polishing pad 108 is statically held in position with respect to the nozzles 116 and the wafer 106 is moved. In another embodiment, the polishing pad 108 is moved in an orbital direction or a linear direction with respect to the nozzles 116. In yet another embodiment, the polishing pad 108 is moved in a bi-linear direction with respect to the nozzles 116. In all cases the wafer 106 may also be moved during the polishing process.

Although various preferred embodiments and the best mode have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention. 

1. A polishing apparatus for polishing a surface of a workpiece comprising: a carrier surface configured to hold the workpiece; a plurality of fluid nozzles placed across from the surface to provide a gap between the nozzles and the surface of the workpiece; and a polishing pad positioned within the gap and configured to polish the surface of the workpiece when a fluid is applied from the plurality of nozzles to push the polishing pad to the surface.
 2. The polishing apparatus of claim 1, wherein an isolation region separates fluid nozzles from one another.
 3. The polishing apparatus of claim 2, wherein the isolation region drain the fluid after the step of applying the fluid to the polishing pad.
 4. The polishing apparatus of claim 1, wherein the fluid nozzles are arranged into a plurality of zones.
 5. The polishing apparatus of claim 1, wherein isolation regions separate zones from one another.
 6. The polishing apparatus o f claim 4, wherein the plurality of zones are circular and disposed concentrically.
 7. The polishing apparatus of claim 4 further comprising at least one regulator associated with each zone and configured to modify the fluid flow from the nozzles to create a desired fluid flow rate profile.
 8. The polishing apparatus of claim 1 wherein the fluid nozzles are configured to be moved close to or away from the polishing pad.
 9. The polishing apparatus of claim 1, wherein the polishing pad is configured to be moved with respect to the fluid nozzles.
 10. The polishing apparatus of claim 1, wherein the polishing pad is configured to be moved bilinearly.
 11. The polishing apparatus of claim 1, wherein the fluid nozzles are placed above the surface of the workpiece.
 12. The polishing apparatus of claim 1, wherein the polishing pad is a fixed abrasive pad.
 13. The polishing apparatus of claim 1, wherein the polishing pad is a polymeric pad.
 14. The polishing apparatus of claim 1, wherein the surface of the wafer comprises copper.
 15. The polishing apparatus of claim 1, wherein the fluid is air.
 16. A method of polishing a surface of a workpiece surface using a polishing pad, the method comprising the steps of: placing the polishing pad within a gap defined between the surface of the workpiece and an array of nozzles; emitting a fluid from the array of nozzles to push the polishing pad onto the surface of the workpiece; and polishing the surface with the polishing pad while keeping the gap constant.
 17. The method of claim 16 further comprising the step of removing the fluid through the isolation space between the array of nozzles.
 18. The method of claim 16, wherein the step of polishing comprises establishing a relative motion between the polishing pad and the surface.
 19. The method of claim 16 further comprising the step of reducing the gap to increase the material removal rate from the surface.
 20. The method of claim 19 further comprising the step of continuing polishing the surface with the polishing pad after the step of reducing the gap.
 21. The method of claim 19, wherein the step of reducing the gap is performed by moving the array of nozzles close to the surface.
 22. The method of claim 19, wherein the step of reducing the gap is performed by moving the surface close to the array of nozzles.
 23. The method of claim 16 further comprising the step of increasing the gap to decrease the material removal rate from the surface.
 24. The method of claim 23 further comprising the step of continuing polishing the surface with the polishing pad after the step of increasing the gap.
 25. The method of claim 23, wherein the step of increasing the gap is performed by moving the array of nozzles away from the surface.
 26. The method of claim 23, wherein the step of increasing the gap is performed by moving the surface away from the array of nozzles.
 27. The method of claim 18, wherein the array of nozzles includes a plurality of zones and the step of emitting fluid comprises emitting fluid with different flow rates from each zone.
 28. The method of claim 27 further comprising the step of removing the fluid through the isolation space between the zones.
 29. The method of claim 16 further comprising delivering a polishing slurry between the surface of the workpiece and the polishing pad.
 30. The method of claim 18, wherein the step of establishing a relative motion includes moving the polishing pad bi-linearly over the array of nozzles while rotating the surface of the workpiece on the polishing pad.
 31. The method of claim 16 further comprising the step of increasing the flow rate of the fluid to increase material removal rate from the surface.
 32. The method of claim 16 further comprising the step of decreasing the flow rate of the fluid to decrease the material removal rate from the surface.
 33. The method of claim 27, wherein the step of emitting fluid with different flow rates from each zone comprises emitting fluid with a flow rate from a zone and emitting fluid with another flow rate from another zone. 